I own a Beneteau First 260 Spirit which has twin rudders mounted on the transom. My question is has anyone any advise on setting their angles and how to ensure they are both angled the same? I am also finding a humming coming from the hull at approx 5 knots - any thoughts?

I have phoned up a few Beneteau dealers and asked their advice. They have all said that the rudders should be set as standard and be straight. Is this not the case with yours?

If you can give me some more information into why they need to be set - I can see what I can do, other wise you might want to call the Ancasta after care team on 0238 060 4670 for some more information.

Hi Roxy

To try to eminiate the hum / vibration I removed the pins which hold the rudders ont the transom and replaced them with s/s nut and bolts at the same time I checked the distance between the edge of each rudder with the edge of the transom to ensure they were both parallel. What I was wondering did I do the right thing or is there some other means to check the alignment / angles of the rudders?

When Beneeau dealers say Standard - what exactly do they mean?

Spoke to a technician and he said the hum is a common problem. The only way to get rid of it is to fair the rudders and he said you can do this in one of two ways -

Either a technician can do it by eye and fixes the fault for you, or knock off a 45 angel on the trailing edge to reduce/get rid of the cavitations (not quite sure what all of that means though).

Got all of that info from calling the number I posted above.

I suspect you mean "angle" although I am intrigued by the idea of knocking off the humming "Angel" ;-)

you knew waht I meant! LOL

Yes of course I was only kidding but I could just picture the Humming angel on the rudder than needed to be got rid of! :-)

Resonance in any system occurs when the applied forcing frequency is the same as or a factor of the resonant frequency of the object (in this case the rudder)

The resonant frequency of the object is related to its mass,stiffness and damping, so modifying any one or more of these factors will alter the resonant frequency. THe trick is to alter the resonant frequency of the object outside any frequencies applied to it or alternatively modify the applied frequencies.

For example the vibration of the new millenium bridge across the Thames between St Pauls and the Tate Modern was cured by adding dampers and weights.

Now that you've probably got the boat out of the water you might like to have a look at the trailing edges of your foils, as this is probably where the hum is coming from. Chris is right in that you could change the frequency ie the speed at which the hum occurs, but if you can eliminate the vortex shedding then you eliminate the cause so won't have it at all. You say the hum is coming from your hull, so it sounds like its the trailing edge of the keel that needs attention - the trailing edge should be square or angled with sharp edges, not round.

That's really interesting Nick! Why are sharp edges preferable to round on the trailing edge of the keel?

I seem to recall adding winglets to aeroplane wingtips or yacht keels help to reduce and shed vortices but don't know how these work.

Chris,
The problem relates to vortex streets, and their generation at the trailing edge of a foil. There's a pretty good explanation here:
http://www.fluidmech.net/gallery/vortex/karman.htm
If you have a round trailing edge, the flow is free to move back and forth as the vortices are formed and shed. With a sharp trailing edge, or a square trailing edge, the flow can only leave the foil at one point, so you don't get the vortex streets being generated.
Winglets work in a slightly different way. On a lifting foil you have a low pressure side and a high pressure side and this results in a tip vortex being generated. The winglets are designed to generate a vortex going in the opposite direction, so reduce the loss of lift at the tip, and so increase the effective span of the foil.
Hope this helps, Nick.

Thanks Nick clear and succinct. Now I must get out the reprofiling kit!